An imbalance of electric charges within or on the surface of a material creates static electricity. This charge imbalance is most commonly observed as resulting from what is known as the triboelectric effect, also referred to as triboelectric charging. Tribolectric charging causes materials with weakly bound electrons to lose electrons through friction to materials with sparsely filled outer shells, resulting in one material becoming positively charged and the other negatively charged. Electrostatic discharge (ESD) is the sudden flow of electricity between two objects caused by contact. In everyday life, a common example of triboelectric charging occurs when someone walks across a floor creating a buildup of static electricity, and a common example of ESD occurs when that person touches a light switch or other conductive material, sometimes resulting in a small spark.
The spark created in the example above is typically harmless, and sometimes even imperceptible, to human beings but can potentially be very damaging to electronic devices and components. The example above of a person walking across a floor and touching a conductive material is just one of many examples of how static electricity can buildup and result in ESD. To prevent damage caused by ESD, electronics manufacturers often include ESD protection circuitry in electronic devices and components such as integrated circuits (ICs) and printed circuit boards (PCBs). An IC with a ground pin and cascaded voltage pins (i.e. pins with different supply voltages), for example, may include ESD protection circuitry to protect the functional circuits between the various pin combinations from both positive and negative ESD stresses. One type of ESD protection circuitry commonly used in electronic devices is an ESD clamp. Upon detecting a voltage event across two pins (e.g., an overvoltage or voltage spike that exceeds a threshold) caused for example by an ESD event, the ESD clamp directs current caused by the voltage event away from functional circuitry, for example to a ground.
ESD protection circuitry adds to overall circuit complexity and requires physical space on the circuit but may be necessary in some cases to protect the functional circuitry of the circuit. Without ESD protection circuitry, circuit reliability is potentially reduced, and the need for time consuming and costly circuit replacement is potentially increased. ESD protection circuitry may influence electromagnetic capability (EMC) performance. Since ESD protection circuitry may not only respond to an ESD event, but to any kind of disturbance of the line connected to the protected pad, the overall EMC performance may be drastically reduced.